An object navigation device is used to track the motion of, for example, a finger or fingerprint. Fingerprint navigation is done by illuminating the finger or fingerprint and tracking its motion across an object surface. As the finger moves across the surface, the motion is detected as a difference of two images recorded by an image sensor over a relatively short time frame. The light reflected by the illuminated finger passes through one or more optical lenses, for example, to change the magnification factor of the image. However, the use of optical lenses results in aberrations, comas, or other image distortion, in particular at the edges of the image. An aperture, or central stop, can be used to correct the aberrations, or distortion, as the light passes through the lenses.
FIG. 1 depicts a conventional air lens structure 10. In particular, the air lens structure 10 is a structure with an immersed medium separating a pair of transmissive surfaces. The air lens structure 10 includes an object surface 12 and an image surface 14. The air lens structure 10 also includes a first portion 24 with a first transmissive surface 16 and a second portion 26 with a second transmissive surface 18. The first transmissive surface 16 and the second transmissive surface 18 are separated by a converging air lens 22, which refracts light at the interfaces with the first and second transmissive surfaces 16 and 18. An aperture or central stop 20 is disposed in the air lens 22 between the first transmissive surface 16 and the second transmissive surface 18. Light reflected by an object at the object surface 12 passes through the first portion 24 and into the air lens 22, which projects the light through the aperture stop 20. The aperture stop 20 controls aberrations from the image as the light travels out of the air lens 22 and into the second portion 26 where the light is focused onto the image surface 14.
The conventional air lens structure 10 has two separate portions 24 and 26 and an embedded aperture stop 20 within the immersed air lens 22. This configuration of the conventional air lens structure 10 consumes a significant amount of space because of the linear alignment of the optical components, as well as the space required for the insertion of the aperture stop 20 to achieve the desired effect of controlling aberrations in the resulting image. The size of the conventional air lens structure 10 restricts the ability to use the structure in different applications. Additionally, embedding the aperture stop 20 between the first and second transmissive portions 24 and 26 complicates the manufacturing process and, hence, increases cost because the first and second portions 24 and 26 are typically manufactured as separate pieces and are then assembled together.